The present invention relates to a system and method for monitoring of at least one parameter associated with the performance of a heart. More particularly, the present invention relates to a system of intrabody implantable sensors with which the performance of a heart can be monitored at any given time point along a prolonged time period.
Chronic heart failure (CHF) results from a deterioration in heart function and is characterized by patterns of hemodynamic, renal, neural and hormonal responses.
The physiological disturbances in congestive heart failure are complex, but a common feature is impairment in the performance of the heart as a pump.
Congestive heart failure is typically brought on by left ventricular dysfunction (LVD). LVD is a condition characterized by impairment in the function of the left ventricular muscle. As a result of this impairment, a reduced cardiac output (which is the product of heart rate multiplied by stroke volume), and an increase in filling pressure are experienced. Left ventricular dysfunction is typically brought on by diseases such as ischemic heart disease, hypertensive heart disease, valvular heart disease, cardiomyopathies and myocarditis. Right ventricular dysfunction (RVD), is a condition of the right ventricle of the heart which is similar to LVD. RVD is usually secondary to LVD, but occasionally can result from a pulmonary disease, valvular pulmonic stenosis, or direct damage to the right ventricular myocardium.
A range of compensatory mechanisms are activated in response to congestive heart failure, which mechanisms are aimed at correcting the reduced cardiac output. These include intrinsic cardiac mechanisms such as, muscle stretching (to increase contractile power), hypertrophy (increase in muscle mass), and a change in the shape of the ventricle. Additionally, a neuro-endocrine response is evoked, leading to an adrenergic increase in heart rate and contraction force, activation of the Renin-Angiotensin-Aldosterone-System (RAAS) which induces vasoconstriction, fluid retention, and redistribution of blood flow. While the neuro-endocrine response is compensatory, it tends to overload the cardiovascular system, leading to additional myocardial damage, and eventually congestive heart failure.
The diagnosis of CHF is achieved by a combination of non-invasive and invasive procedures. These procedures include physical examination, electrocardiogram (ECG), blood tests, chest radiography and echocardiography. Additional tests may be prescribed if needed to establish the presence of the disease and it's etiology.
Management of a CHF patient is often an enormous challenge to the treating physician. The compensatory mechanisms evoked by heart failure, make it necessary to provide treatment at a broad front, which typically requires the balancing of several potent drugs. At times this treatment is thwarted by the compensatory mechanisms, which recompensate for the presence of the medical treatment. Thus, the medical treatment of CHF is more of a "disease management" task than a "treatment", and in essence relies upon balancing the hemodynamic status of the patient in a state of compensation, such that the progression of CHF is kept to a minimum.
The management of CHF also includes non-medical intervention such as exercise to the extent tolerated, weight control, sodium restriction, and abstinence from smoking and alcohol.
The delicate balance between compensation and effective treatment is easily upset, even by seemingly benign factors, such as common medication (e.g., aspirin), physiological factors, excitement, or gradual progression of the disease. This may plunge the patient into a decompensation crisis, which requires immediate corrective action so as to prevent the deterioration of the patient's condition which, if left unchecked, can lead to death.
Central to disease management is constant patient monitoring. Presently, the most commonly used monitoring devices, monitor parameters which indirectly indicate the hemodynamic status of the patient. However, different hemodynamic situations, which require radically different corrective actions, may present similar clinical findings and as such a correct treatment regimen cannot always be prescribed by a physician. Monitoring of more direct hemodynamic measures requires more complex procedures, such as echocardiography or invasive methods. These procedures are usually employed only when a hemodynamic crisis has developed, or when trial and error treatment has failed to produce the desired results. An additional obstacle to proper patient management is the frequency of monitoring. A hemodynamic crisis may develop in a matter of hours or days. Alternatively, the patient may go through a phase where the hemodynamic parameters slowly change into a decompensatory state in which case monitoring over a long period of time is required in order to detect the onset of such a change.
There are various methods to measure heart performance and as such to detect CHF. These methods are categorized as non-invasive or invasive measurement methods.
Non-invasive measurement methods obtain information which relates indirectly to the performance of the cardio-vascular system.
For example, measurements of lung partial oxygen pressure (pO.sub.2) at exhale can be exercised to monitor the state of a patient. Such measurements are cheap to perform and are non-invasive by nature, but the results are not considered a valid measure of the cardiac status in CHF unless corroborated by another independent measurement. Another method to determine pO.sub.2 relies upon the metabolic rate ratio (see, for example, U.S. Pat. Nos. 4,909,259 and 5,836,300). The concentrations of oxygen and carbon dioxide in the breath of the patient are measured. The data obtained by these measurements also includes the effects of cardiac output (CO), lung problems and various other metabolic parameters and, as such this method requires additional data to be accurate.
Attempts have also been made to derive the principal CO parameters from ECG waveforms using an experimental model which relates stroke volume with ECG electrical parameters (see, for example, U.S. Pat. Nos. 5,025,795 and 4,854,327). Although these methods can easily be applied to long-term disease management, the lack of data pertaining to the significance and validity of the experimental model and its dependence on stress, past cardiac events and other parameters is not provided and as such these methods cannot be considered accurate. Moreover, none of these methods can be used to determine with accuracy the left ventricle filling pressure which is a crucial parameter for proper patient management.
The condition of a CHF patient can also be assessed by the heart size. This can be accomplished non-invasively by either x-rays or by an echo-cardiography, although in both cases highly accurate readings cannot be collected.
U.S. Pat. No. 5,178,151 discloses a non-invasive heart function monitor which measures cardiac volumes from torso movements in two planes combined with ECG measurements. An elaborate model is used to extract these volumes from the data. A highly skilled staff is required to assess the results of this procedure.
U.S. Pat Nos. 5,469,859; 5,735,284 and 4,450,527 all describe a non-invasive bio-impedance devices which employ two to four thoracic or peripheral electrodes utilizable in determining a patient's cardio-respiratory parameters. Although this approach can be used for long term disease management due to its simplicity and non-invasiveness, it suffers from several drawbacks. For example, the results obtained depend on perfect electrical coupling to the body at precise locations. In addition, since the electrical path in the human tissue is complex and varies with the individual and his/her breath regimen, the method yields semi-accurate results which oftentimes have to be correlated to other measurement methods. Finally, due to its non-specificity, the obtained CHF related results may be contributed by other edemic processes (e.g., in the lungs).
Although non-invasive procedures can be used to asses the condition of a patient suffering from CHF, such procedures do not provide the level of accuracy which can be achieved by invasive methods. Invasive methods typically use wiring and catheters to extract information directly from the heart to be externally processed, but since these methods can be hazardous to the patient they can only be implemented for short-term monitoring procedures (hours to a few days) or during surgical procedures. Such invasive methods are generally utilized in New York Heart Association (NYHA) class III and IV patients.
Several parameters can be measured via invasive procedures in order to determine cardiac performance. These parameters include "cardiac output" (CO), the "left ventricular end diastolic pressure" (LVEDP), "left ventricular filling pressure" (LVFP), "aortic flow", (AF), "pulmonary arterial pressure" (PAP), "pulmonary arterial flow" (PAF), "myocardial contractility" (MC) and heart size. Cardiac output is defined as the volume of blood pumped by the heart in one minute, and is the product of stroke volume multiplied by the average pulse rate per minute. LVFP can be measured either in the left atrium when the atrial valve is open, (ending in the closing of the mitral valve at the LVEDP), or in the pulmonary arterial trunk using the "wedge" method. Myocardial contractility can be measured as the time derivative of the left ventricular pressure, measured at the isovolumic contraction phase of the systolic cycle. The ejection fraction, EF, which is the ratio of stroke volume to the end-diastole left ventricular volume is considered another index of systolic ventricular function.
LVEDP and LVP are difficult to measure directly. The left ventricular flow is remarkably different than that of the right ventricle, being oftentimes characterized by a turbulent flow. Blood emboli formed due to such an invasive procedure, can migrate to the brain via the carotid artery. Furthermore, due to hazards associated with such placement of a catheter, long-term positioning in the left ventricle (LV) or left atrium (LA) is not typically employed, and thus the possibility of long-term monitoring is significantly limited. Since non-invasive echo measurements of the heart size and contractility are not always considered sufficient, myocardial contractility (MC) and heart size data can only be efficiently acquired through invasive procedures attaching position and acceleration sensors to the myocardium, or by using a pressure catheter inside the LV.
One invasive measurement method of CO requires the patient to breath pure O.sub.2 from a spirometer with a CO.sub.2 absorbr, while measuring the O.sub.2 uptake directly from the net gas flux. Two catheters, one in the pulmonary artery (PA) and one in the brachial or frmoral artery are used to simple the mixed bloodstream. While theoretically this method can achieve a high degree of accuracy, this technique suffers from several inheret limitations which limit it's accuracy and efficiency. The patient cardiac output and O.sub.2 consumption must be constant over several minutes for an accurate measurement to be taken; more than one catheter is required to effect measurements, which increases the risk of infection; a significant volume of blood is used by the test, rendering it inconvenient for repeated measurements, especially on infants or acutely ill patients; and, the monitors used to analyze gas concentration are often bulky, and/or located remote from the patient, making real-time analysis difficult or impossible. U.S. Pat. No. 5,040,538 to Mortazavi discloses a method which is a modem and invasive variant to pO.sub.2 blood measurements. According to this method, pO.sub.2 is measured using an optical sensor installed on the sensor side of a pacemaker catheter in the right atrium or ventricle. Oxygenation level is measured using a dual-LED sensor. A first LED illuminates the bloodstream while a second LED senses the reflected light from the blood, thus setting a pre-calibrated level of oxygen. The disclosed sensor output is sufficient to control the rate of a pacemaker, but may not be adequate for CO determination since the sensor level determination is not related in any form to O.sub.2 consumption.
Of all the measurable hemodynamic parameters, LVEDP is considered the most valid and informative. Since a direct measurement of LVEDP is considered very risky, a wedge-pressure, which is typically acquired via a Swan-Ganz (SG) catheter, is often obtained as a representative parameter to LVEDP. Preferably, the catheter is introduced through a central vein, into the right atrium. It is maneuvered into the right ventricle and into the PA and positioned at the PA trunk. A balloon is inflated to block the flow in one of the PA branches, and the "wedge" pressure is taken at its distal end. The assumption is that since the blood flow is blocked, this value is equilibrated with and equals to the LA pressure, which in turn is equal to LVFP. A flow measurement is then taken by thermodilution in the PA trunk. The pressure and the flow are very close to their instantaneous values and waveforms in the LA. Although SG catheterization is not considered risky, a migration of the catheter into the branch of the PA may cause a pulmonary infarct. SG is, therefore, a proper tool only for acute CHF and cannot be used for extended disease management. Another approach which is described in U.S. Pat. No. 5,755,766 is to implant a pressure sensor in the great coronary vein situated on the myocardium, posterior to the LV, and measure approximate waveforms of the LVP.
A variation on the SG pressure and flow measurement, which utilizes an SG catheter, is the thermodilution method. In this method, the SG catheter is inserted, and a small prescribed volume of solution, which is colder than the blood, is instantaneously injected into the RA. Simultaneously, temperature versus time is measured at the PA downstream. Since both volume and temperature of the diluting sample, as well as the patient temperature are known, it is possible to obtain the flow values in terms of volume (see, for example, U.S. Pat. No. 5,277,191). U.S. Pat. No. 5,509,424 describes another device and method equivalent to the original SG catheter, but using a heated instead of a cooled sample. A mildly invasive method used to measure flow speed is Trans-esophageal echocardiography, (TEE) (see for example U.S. Pat. Nos. 5,052,395 and 5,022,410). An ultrasonic catheter is introduced through the esophagus down to the region of the descending aorta. Since the ascending aorta and the esophagus run almost parallel to each other, Doppler echo measurement of the flow speed in the aorta are difficult to obtain. Although the procedure is only mildly invasive, it is lengthy, cumbersome, expensive, unpleasant, and is usually performed in a hospital. In addition, the quality of the results is highly dependent on the operator skills. The method is, therefore, not appropriate for extended CHF management.
There is thus a widely recognized need for, and it would be highly advantageous to have, a measurement method which can be used to monitor hemodynamic parameters of a CHF patient over an extended period of time. Such a method enables to determine the hemodynamic status of a patient and therefor will enable optimizing a suitable therapy regimen accordingly. Using such method, disease management of for example, NYHA type III and type IV patients would be greatly simplified thus helping improve life expectancy in such individuals.